COMPE 438 JAVA PROGRAMMING

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COMPE 438JAVA PROGRAMMING

• F. Cemile Serçe

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Exceptions

• In Java error exception
• Programs can generate errors
• Arithmetic
• Divide by zero, overflows,
• Object / Array
• Using a null reference, illegal array index,
• File and I/O
• Nonexistent file, attempt to read past the end of
  the file, (well see more about file I/O later in
  course),
• Application-specific
• Errors particular to application (e.g., attempt
  to remove a nonexistent customer from a database)

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Exceptions
void readFile String fileName
c\a.txt openFile(fileName) findSize(fileName
) allocateMemory(fileName) readFileFromMemory(
fileName) closeFile(fileName)
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Exceptions

• unexpected conditions in programs
• approaches for dealing with error conditions
• Use conditional statements and return values
• Use Javas exception handling mechanism
• java exception handling facilitates recovery from
  unexpected condition or failures
• you can restore program stability (or exit
  gracefully)
• sources of exception
• Run-time environment, including java class
  library
• Java programs

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Exceptions(cont.)

• Hierarchy of Exceptions

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Exceptions(cont.)

• Unchecked Exceptions
• represent defects in the program (bugs) - often
  invalid arguments passed to a non-private method.
  (errors in your program's logic)
• are subclasses of RuntimeException, and are
  usually implemented using IllegalArgumentException
  , NullPointerException, or IllegalStateException
• a method is not obliged to establish a policy for
  the unchecked exceptions thrown by its
  implementation (and they almost always do not do
  so)

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Exceptions(cont.)

• Checked Exceptions
• represent invalid conditions in areas outside the
  immediate control of the program (invalid user
  input, database problems, network outages, absent
  files)
• are subclasses of Exception
• a method is obliged to establish a policy for all
  checked exceptions thrown by its implementation
  (either pass the checked exception further up the
  stack, or handle it somehow)

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Exceptions(cont.)

• Throwable is the superclass of all errors
• different categories of Throwable
• Error
• serious and fatal problems in programs
• thrown by JVM, not handled by regular programs
• Exception
• thrown by any program
• user-defined exceptions are subclasses of
  Exception
• RuntimeException
• subclass of Exception
• caused by illegal operations
• thrown by JVM

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Exceptions(cont.)

• Throwing Exceptions
• throw exception anywhere in a program
• use throw statement

throw Exception
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Exception Behaviour

• If program generates (throws) exception then
  default behavior is
• Java clobbers (aborts) program
• Stack trace is printed showing where exception
  was generated

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Exception Behaviour

• Example

public static int findIndex(int ints, int
s) for (int ret 0 ret lt ints.length
ret) if (intsret s) return ret return
-1
Consider
int ints 0, 2, 1, 5
Evaluating findIndex(ints, 6) results in
Exception in thread "main" java.lang.ArrayIndexOut
OfBoundsException 4 at exceptionExamples.SearchEx
ample.findIndex(SearchExample.java13) at
exceptionExamples.SearchExample.main(SearchExample
.java27)
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Java Exception Terminology

• When an anomaly is detected during program
  execution, the JVM throws an exception
• There are built-in exception
• Users can also define their own (more later)
• To avoid crashing, a program can catch a thrown
  exception
• An exception generated by a piece of code can
  only be caught if the program is alerted.
• This process is called trying the piece of code

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Java Exception Terminology

• Every exception object has the following methods
• Exception(String message)
• Constructor taking an explanation as an argument
• String getMessage()
• Returns the embedded message of the exception
• void printStackTrace()
• Prints the Java call stack when the exception was
  thrown

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Examples from Java library

• ArithmeticException Used e.g. for divide by zero
• NullPointerException attempt to access an object
  with a null reference
• IndexOutOfBoundsException array or string index
  out of range
• ArrayStoreException attempting to store wrong
  type of object in array
• EmptyStackException attempt to pop an empty
  Stack (java.util)
• IOException attempt to perform an illegal
  input/output operation (java.io)
• NumberFormatException attempt to convert an
  invalid string into a number (e.g., when calling
  Integer.parseInt( ) )
• RuntimeException general run-time error
  (subsumes above)
• Exception The most generic type of exception

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Handling Exceptions in Java

• Java uses try-catch blocks for exception
  handling. Syntax

try catch (ltexntype1gt e2)
catch(ltexntype2gt e2) finally //
finally is optional
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Example
public static void main(String args) int
ints 0, 2, 1, 5 try System.out.print
("Searching for 6. ") System.out.println ("Index
is " findIndex(ints, 6)) catch
(IndexOutOfBoundsException e) System.out.println
("\nThere must be an array error.") System.out.p
rintln ("Message is " e.getMessage())
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Exception Propagation

• Java uses exception propagation to look for
  exception handlers
• When an exception occurs, Java pops back up
  the call stack to each of the calling methods to
  see whether the exception is being handled (by a
  try-catch block). This is exception propagation.
• The first method it finds that catches the
  exception will have its catch block executed.
  Execution resumes normally in the method after
  this catch block
• If we get all the way back to main and no method
  catches this exception, Java catches it and
  aborts your program

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Throwing Exceptions in Java

• To throw an exception, use throw command throw
  e
• e must evaluate to an exception object
• You can create exceptions just like other objects
• RuntimeException is a class
• Calling new this way invokes constructor for this
  class
• RuntimeException generalizes other kinds of
  exceptions (e.g. ArithmeticException)

RuntimeException e new RuntimeException(Uh
oh)
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Example
public class EasyExample public static void
g(boolean b) System.out.println ("Entering g
...") if (b) throw new RuntimeException("Oh no,
you really messed up...") System.out.println
("Exiting g ...") public static void f(int i)
System.out.println ("Entering f ...") g(i
0) System.out.println ("Exiting f
...") public static void main(String args)
try f(1) catch (RuntimeException e)
System.out.print ("Runtime error. Message
") System.out.println (e.getMessage())
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Example
public class EasyExample public static void
g(boolean b) System.out.println ("Entering g
...") if (b) throw new RuntimeException("Oh no,
you really messed up...") System.out.println
("Exiting g ...") public static void f(int i)
System.out.println ("Entering f ...") g(i
0) System.out.println ("Exiting f
...") public static void main(String args)
try f(0) catch (RuntimeException e)
System.out.print ("Runtime error. Message
") System.out.println (e.getMessage()) System.
out.println ("Exiting g ...")
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Exceptions(cont.)

• Examples

public FileReader(String fileName) throws
FileNotFoundException
class OpenFile public static void
main(String args) if (args.length
gt 0) try
// Open a file
FileReader f new
FileReader(args0)
System.out.println(args0
" opened")
f.close() catch (IOException x)
System.out.println(x)

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Exceptions(cont.)
class Inventory private int stockLevel
0 public boolean addToInventory (int
amount) final int MAX 100 int temp temp
stockLevel amount if (temp gt
MAX) System.out.print("Adding " amount "
item will cause stock ") System.out.printl
n("to become greater than " MAX "
units(overstock)") return false else stockLe
vel stockLevel amount return true //
End of method addToInventory
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Exceptions(cont.)
public void addToInventory (int amount) throws
InventoryOverMaxException int
temp temp stockLevel amount if (temp gt
MAX) throw new InventoryOverMaxException
("Adding " amount " item
will cause stock to become greater than "
MAX " units")
class InventoryOverMaxException extends
Exception public InventoryOverMaxException
() super () public InventoryOverMaxException
(String s) super (s)